High frequency electronic integrated circuits (IC) used in modern electronic communications and computing devices require high bandwidth amplifiers. Furthermore, the low voltages used in modem IC designs lead to a need for high bandwidth amplifiers having a high transconductance, that is, amplifiers capable of generating significant change in output current for a relatively small change in input voltage. Such devices ideally should also have high input impedance and a choice of high or low output impedance. And, because modem electronic devices are increasingly portable, battery-powered devices, it is also highly desirous that the high bandwidth amplifiers be as energy efficient as possible.
Traditionally, source follower amplifiers have been used to provide high bandwidth amplifier outputs. These circuits are well known in the art and are typically used to buffer signals and provide low output impedance circuits capable of driving loads at high frequency. However, traditionally designed source followers have relatively low transconductance, which limits their ability to drive loads, especially in modem, low voltage IC design. Traditional source followers also have the limitation that they are class A type amplifiers, with a maximum load current limited to the quiescent current in the buffer. Having a type A amplifier with large quiescent current is inherently energy inefficient.
Previous attempts to increase the load drive capability of low voltage source followers have focused on sensing the current in the source follower drain and folding it back to increase the effective transconductance of the device. These prior art high bandwidth, low voltage gain cells include, for instance, the circuits described in U.S. Patent Application Publication US 2002/0175761 A1 titled “High-Bandwidth Low-Voltage Gain Cell and Voltage Follower Having an Enhanced Transconductance” by Bach et al, the contents of which are hereby incorporated by reference.
These circuits achieve the required low voltage operation with high bandwidth and high transconductance, but operate essentially in a class A type mode, drawing significant current even when there is no input signal. Moreover, such circuits are limited in drive capability in one direction by the quiescent current.
To improve power consumption, there is a need for a high bandwidth, high transconductance source follower circuit that operates in a class AB mode, in which only a small current is drawn in the absence of an input signal and in which the drive current is not limited by the quiescent current in either the sink or source direction.